1. Field of the Invention
This invention relates to analysis of materials by means of time-of-flight medium energy particle scattering. More particularly, the invention is directed towards a time-of-flight technique which is sensitive to both ions and neutral atoms.
2. Background Information
The method and apparatus of the present invention is directed toward surface analysis by Medium Energy Particle Scattering (MEPS). The invention relates to a novel method and related apparatus for time-of-flight measurement of particle velocities for use with medium energy particles. "Medium Energy Particles" as used herein means particles having an energy of between 10 keV/u and 200 keV/u where a keV/u is a kilo-electron-volt per atomic mass unit.
Prior art techniques for determining the composition of a surface include: Auger Electron Spectroscopy (AES), Secondary Ion Mass Spectrometry (SIMS), Low Energy Ion Scattering (LEIS), also called Ion Scattering Spectrometry (ISS), charged-particle sensitive (energy or momentum dispersive) Medium Energy Ion Scattering (MEIS), and Rutherford Backscattering Analysis (RBS). In each of these techniques, a probe beam of particles is directed onto the surface to be investigated. The interaction of one of these primary beam particles with the atoms of the surface material causes either the emission of a particle or the reflection of the primary particle. The analysis of this emitted or scattered particle's properties gives information about the surface.
It is known that RBS most directly provides reliable quantitative information about the composition of the surface under study. This is because RBS depends upon the Coulomb interaction between the nucleus of the primary ion and that of the surface atom with which it collides. This makes RBS much less sensitive to the chemical environment of the surface than other techniques, almost all of which depend upon atomic phenomena for their effectiveness. The technique of the present invention employs the Coulomb interaction, while attaining the much higher cross-section associated with lower energy scattering as discussed in more detail hereinafter.
Prior art time-of-flight detector designs include particle telescopes used for mass identification of particles emitted in nuclear reactions and occurring in the ambient space environment (See Gloeckler et al., "Time of Flight Technique for Particle Identification at Energies from 2-400 keV/Nucleon", Nucl. Instr. and Methods 165(1979) 537-544; U.S. Pat. No. 4,611,118, U.S. Pat. No. 4,072,862 and other mass spectrometers such as U.S. Pat. No. 4,818,862.)
Various other time-of-flight systems have also been known. See U.S. Pat. No. 4,831,254 for an ion drift detector, and U.S. Pat. No. 4,677,295 for a plasma desorption time-of-flight detector.
None of these prior art methods provide high-resolution measurement of the velocities of charged and neutral particles in the medium energy range (as defined above). In general, the detectors in widespread use today are solid state Si surface barrier detectors. These devices are simple, accurate and inexpensive, but even the best have an alpha particle energy resolution of about 10 keV. This relatively high value sets a lower limit on the energy of the beam of several hundred keV. Electrostatic detectors provide high resolution in this energy range, but are only capable of detecting ions.
There remains a need for a detector which is equally sensitive to both scattered ions and neutrals and which will allow reliable particle scattering analysis using particle beams with lower energies. There is a need for detectors providing the same precision, speed, ease of use as found in the conventional RBS methods and offering increased sensitivity, surface specificity and depth resolution. There is also an important need for a quantitative surface analytical technique which can be carried out using accelerators capable of providing particles in the medium energy range, as such accelerators are much more widely available and less costly than those which provide high energy particles.